The invention relates generally to the field of fiber optic directional couplers, and more specifically, to devices which couple light energy between the polarization modes of an optical fiber.
It is useful to be able to convert light traveling in an optical fiber from one waveguide to another dissimilar waveguide, i.e., one with different propagation characteristics. If this transfer can be caused by some physical phenomena, a sensor can be made. In fiber optics, a single birefringent fiber can be thought of as two dissimilar waveguides in that such fiber can guide light and maintain its polarization in either of two independent polarization modes. To be able to control power transfer between these two modes is highly desirable.
Such coupling in the microwave art has been achieved by placing dissimilar waveguides side by side and drilling holes in the common wall at a periodic spacing equal to the beat length. The beat length is the distance it takes two signals of the same frequency traveling at different velocities in different waveguides, or at different modal velocities in the same waveguide, to shift 360 degrees in relative phase. The holes in the waveguides are spaced at the beat length causing additive coupling to occur between waveguides resulting in power transfer. This result follows from a principle of quantum mechanics called conservation of momentum.
Similar results have been obtained in integrated optics where lithium niobate crystals with diffused titanium waveguides have electrodes placed on the waveguide at a spaced equal to the beat length. Lithium niobate is naturally birefringent. Electric potential of alternating polarity is applied periodically every half beat length by the electrodes which causes changes in the axes of birefringence by electroptic effect in the waveguide. These changes may or may not be abrupt, but probably are not abrupt. The abruptness of the changes in the birefringence axes is important to substantial power transfer. The change in the properties of the material caused by the alternating electric fields causes coupling of power from one polarization mode to the other at each point of perturbation.
Such lithium niobate structures are lossy however and have not exhibited transfer ratios of greater than 25 dB. Losses can be as much as 1 dB in the crystal. Further losses occur at splices between fiber waveguides and the lithium niobate at both the input and output ports. Such losses can impair the performance of the system in which the crystal is used. Further, the beat length of lithium niobate is on the order of a few hundred microns, so complex photolithography techniques must be used for deposition of the electrodes.
Accordingly, a need arose for a simple, all fiber device which can control the transfer of power between polarization modes.